Synchrotron-Based XANES Speciation of Chromium in the Oxy-Fuel Fly Ash Collected from Lab-Scale Drop-Tube Furnace

Speciation of chromium (Cr) in the fly ash collected from oxy-firing of Victorian brown coal has been reported for the first time to address the potential formation of toxic Cr(VI) and the variation of the quantities of Cr(III)-bearing species with flue gas composition. Synchrotron-based X-ray absorption near-edge structure (XANES) was employed for Cr speciation. Apart from a pure O2/CO2 mixture (27/73, v/v) versus air, the O2/CO2 mixtures doped with SO2, HCl, and steam individually or together to simulate real flue gas have also been tested. Under all of the conditions tested here, the fractions of Cr(VI) in the fly ashes are insignificant, constituting no more than 5% of the total Cr. The test of Cr-doped brown coal in pyrolysis further confirmed that the Cr(VI) formation preferentially occurred through a local oxidation of Cr(III) at the oxygen-containing functions sites within coal matrix, rather than through an oxidation by external bulk O2. This reaction is also highly temperature-dependent and slower than the interaction between Cr(III) and other metals such as iron oxide. Increasing temperature to 1000 °C inhibited the oxidation of Cr(IIII) to Cr(VI). Shifting the combustion gas from air to O2/CO2 exerted little effect on the Cr(VI) formation. Instead, the formation of iron chromite (FeCr2O4) was facilitated in O2/CO2, probably due to a strong reducing microenvironment formed by the CO2 gasification reaction within the char matrix. The accumulation of HCl in flue gas favored the vaporization of chromium as gaseous chloride/oxychloride, as expected. The coexistence of SO2 inhibited this phenomenon by promoting the formation of sulfate. The presence of steam was even beneficial for the inhibition of water-soluble Cr sulfate through stabilizing the majority of Cr into alumino-silicate which is in the slagging phase.